Sodium dichloroacetate (DCA) impairs β-secretase-mediated proteolysis of the alzheimer's disease amyloid precursor protein (APP)

Hammond, Jessica and Parkin, Edward (2016) Sodium dichloroacetate (DCA) impairs β-secretase-mediated proteolysis of the alzheimer's disease amyloid precursor protein (APP). Masters thesis, Lancaster University.

Abstract

Alzheimer's disease (AD) is caused by the accumulation of neurotoxic amyloid-beta (Aβ)-peptides in the brain. These peptides result from the amyloidogenic cleavage of the amyloid precursor protein (APP) by y-secretase yielding soluble APPbeta (sAPPβ) and a membraneassociated C-terminal fragment, C99. The latter fragment is then cleaved by a-secretase liberating the toxic Aβ-peptides. Alternatively, APP can be cleaved non-amyloidogenically by an α-secretase activity (a member(s) of the a disintegrin and metalloproteinase, ADAM, family of zinc metalloproteinases) which cleaves within the Aβ region of the protein producing soluble APPalpha (sAPPα) and a membrane-associated C-terminal fragment (C83) and precluding the formation of intact Aβ-peptides. In this study we show that, in human neuroblastoma SH-SY5Y cells, the mitochondrial modulator dichloroacetate (DCA) impairs the β-secretase-mediated processing of APP as determined by impaired sAPPβ production. Conversely, the non-amyloidogenic processing of APP was enhanced resulting in more sAPPα production. Over-expressing the β-secretase (BACE1) in SH-SY5Y cells partly reversed this phenomenon as sAPPα production did not significantly increase and sAPPβ751/770 levels were unaltered. Over-expressing APP695 resulted in unchanged sAPPβ generation following treatment with DCA. The effect of DCA on two further ADAM substrates, E-cadherin and Jagged1 was investigated in different cell lines in order to examine the substrate and cell-type specificity of DCA. Endogenous E-cadherin expression in both SW480 colorectal cancer cells and HEK-Jagged1 (HEK cells stably overexpressing Jagged1) was decreased following DCA treatment as was shedding of the protein. Endogenous APP expression and sAPPα shedding was increased in both cell lines, however it was not possible to detect sAPPβ in the samples. Conversely the expression and shedding of endogenous Jagged1 in SW480 cells was enhanced following DCA treatment whilst, in HEKJagged1 cells, expression of the transfected protein was unaltered but its shedding was enhanced. These data indicate that the effects of DCA on APP are not common to all ADAM substrates. Thus, we postulate that the mode of activity of DCA involves the stimulation of APP release from the cell surface by a-secretase resulting in reduced internalisation and subsequent β-secretase-mediated proteolysis. When APP is over-expressed the α-secretase pathway becomes saturated resulting in more internalisation of intact APP and its availability for β-secretase cleavage Given that DCA is a very small molecule already used in humans for the treatment of specific medical conditions and can safely transit the blood brain barrier, it may be a suitable pharmaceutical candidate to delay or treat the development of AD.